[600MRG] RF Ammeter

Sat Nov 21 12:02:43 EST 2020

>Would a clamp-on AC ammeter work for measuring RF currents at 2200M and 630M or does the frequency response of clamp-on/inductive coupling roll off and is no longer useable?

I have seen guys using the lighbulb technique and a clamp on meter to
"proof" a thermo meter but I dunno how much I would trust that
honestly due to different responses at 472 vs. 60 Hz.

If you are going to use the metering that Neil linked, be mindful that
its really not designed to be a permanent "thru-line" meter under the
power levels we typically encounter.. You will likely set that shunt
resistor on fire at 472 with any real power.. It was apparently
presented for use on the Part-15 lowfer freqs if I am re-reading it
properly.  If you do plan on using this approach, put a switch in
parallel to shunt around  meter / rectifier / load when not measuring.

There is another approach on JGV to a remote transformer sample /
meter box that was designed with multiple current levels in mind and
could handle all sorts of crazy power.  That was what I was thinking
about.

Sounds like your meter is going to be just fine, however, based on the
resistance measures,

73/GM

jl

On 11/21/20, Scott Armstrong <aa5am at vntx.net> wrote:
> Thanks for all the  information received on and off the list.  I've got a
> couple usable designs to build something. The challenge now is to find what
> is available in the junk box!
>
> @John
>
> I place a VOM across the terminals of the Simpson meter and resistance is
> equal to or slightly higher than what the lead resistance of the  VOM is.
> Need to check with a more reliable VOM but looks promising thus far.  The
> Simpson meter  will be of more use if/when I get something up in the
> 200-300W range.
>
> Seeing the information that Niel provided and other info found on the web
> about testing with a AC /60Hz source....
>
> Would a clamp-on AC ammeter work for measuring RF currents at 2200M and
> 630M or does the frequency response of clamp-on/inductive coupling roll off
> and is no longer useable?
>
> -Scott AA5AM
>
> On Sat, Nov 21, 2020 at 7:45 AM Neil Klagge <w0yse at msn.com> wrote:
>
>> Scott, here is a link to W5JGV’s projects page  Showing how he made a
>> homemade  meter. I have built several of these.
>>
>> http://w5jgv.com/rfa-2/rfa-2.htm
>>
>> You can calibrate it with AC through 100 W bulb if necessary. Of course
>> be
>> careful with the clip leads when you do that LOL.
>>
>> Neil Klagge
>> w0yse
>> Northern Utah
>>
>>
>> Sent from Neil's iPhone
>>
>> On Nov 21, 2020, at 05:51, John Langridge <kb5njd at gmail.com> wrote:
>>
>> Hi Scott, the way to tell if its a thermo is to put a VOM across the
>> terminals.  If the R is very low or zero, its likely a thermocouple.
>> If you see winding resistance, like 20 ohms or 100 ohms, for example,
>> its just a volt meter with a current scale that will need to be tied
>> to a therm or a current transformer...  I often find that the meters
>> with deeper cases tend to be more likely to be a therm... Also, my
>> experience with buying working thermocouple at hamfess is about 50%..
>> often times they show up with a burned-up junction or coil...
>>
>> W5JGV had a nice transformer-based current meter with remote sampling.
>> I dont have the link handy right now but it may be on my links page...
>>
>> I use both here and monitor antenna current in the ham shack
>> religiously.  I calibrated a volt meter with a current scale to a
>> transformer in the ATU to  a thermocouple ammeter.  Its linear where I
>> use it.  but I find the information far better at determining system
>> health than SWR...
>>
>> as you suggest, your meter may have a hard time at lower current
>> levels.  but determine what it actually is first.  A current
>> transformer and a volt meter can better resolve those low levels if
>> you have an accurate meter to calibrate with.
>>
>> 73!
>>
>> John..
>>
>>
>>
>>
>>
>>
>> On 11/20/20, Scott Armstrong <aa5am at vntx.net> wrote:
>>
>> Hi Guys,
>>
>>
>>
>> A couple of questions....
>>
>>
>>
>> Wondering if anyone has any information on this meter.
>>
>>
>> I have a Simpson Model 135 RF Ammeter.  Full scale is 2.5A
>>
>> Does this meter have a thermocouple built in or did it use an external
>>
>> thermocouple?
>>
>>
>>
>>
>> Can anyone point me to a schematic for a RF Ammeter that will work at
>> 2200m
>>
>> and 630m and has a top end of say 1-1.5 A.
>>
>>
>> The Simpson meter, if it does have a TC built in, will not work very well
>>
>> for low power ~25Wapplication. The meter scale being non-linear  does not
>>
>> have much resolution on the bottom end of the scale around the .7A range.
>>
>>
>> Thanks,
>>
>> Scott AA5AM
>>
>>
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